In order to increase the recording density of a high density magnetic recording medium, there has been used a nonmagnetic support film having a very smooth surface. When the surface of the nonmagnetic support film is smooth, the frictional resistance thereof against a conveying roller becomes large, thereby frequently causing winding or wrinkles. Further, the frictional resistance between the nonmagnetic support films are increased so that a winding roll is liable to be warped.
Recently, using a deposition or sputtering, etc., there has been developed a magnetic recording medium comprising a ferromagnetic thin film made of Co-Ni, Co-Cr, or Fe-N, etc. However, since the ferromagnetic thin film is much more smooth and thin than a magnetic layer formed by coating which comprises mainly ferromagnetic particles and a binder resin, the above-mentioned problems are further increasing.
In order to solve such problems, various attempts have been made, one of which is to provide an intermediate layer of a coating type between a magnetic layer and a support. Depending on the situation, such intermediate layer can be called an under coat.
For example, in JP-A-56--116115 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-53--128683, JP-A-54--94574, JP-A-56--10455, and JP-A-56--16937, the surface of a nonmagnetic support is finely and uniformly roughened so that it can well touch a magnetic head and the running property can be improved.
JP-A-58--68227 discloses a method in which a ferromagnetic thin film is formed on a plastic film, the surface of which has a granular film to which fine particles having a size of 0.01 to 0.2 .mu.m or is formed with worm-like nodules are added.
JP-A-59-48825 proposes that colloidal silica and binders are used to form an undercoating layer, thereby forming 1000 pieces/mm.sup.2 or more of granular protrusions of 30 to 500 .ANG. high.
However, in these prior arts, it is difficult to form on a support an undercoating layer in which fine particles are uniformly dispersed in binders. Namely, there is formed on a support an undercoating layer in which fine particles are not uniformly dispersed but aggregate therein.
As a result, there are problems that clearance loss between a magnetic head and a magnetic layer is increased, thus lowering the output; still life is made to be shortened; and running durability is made to be lowered.